The present invention relates to the manufacture of composite reinforcement elements woven or knitted in three dimensions from textile, mineral, synthetic or other fibers impregnated with a resin which is subsequently polymerized or otherwise hardened.
Reinforcement elements of this type are principally, but not exclusively, employed in the aeronautic and space fields in which they have many applications, in particular for producing parts which must withstand thermo-mechanical stresses, as the thermal protections of bodies which re-enter the atmosphere, the powder rocket nozzles, aircraft brakes, or parts which must withstand high mechanical stresses such as hubs of helicopter rotors, undercarriages and inserts of wings, leading edges, etc.
Many processes and apparatus have been imagined and developed for producing such reinforcement elements, but the automatized manufacture of parts of complex shape encounters great difficulties which result in very complicated and consequently costly machines without the parts obtained always possessing all the required qualities of homogeneity and resistance.
Furthermore, the remarkable properties of these composite elements lead to the use thereof for constructing parts having complicated, evolutive shapes that present machines are incapable of manufacturing.
Machines are known for producing composite reinforcement elements which are of revolution, hollow and woven in two dimensions horizontally around rigid perpendicular rods mounted in concentric ring arrangements on a rotatable support, which are subsequently replaced by threads, as described for example in US-A-4,183,232 and US-A-4,346,741 in the name of the applicant.
According to another method, a hollow support mandrel is used on the surface of which a plurality of parallel layers of threads is deposited in two crossed directions and lines of stitching perpendicular to these layers are formed, as described in particular in FR-A-2,355,936.
According to FR-A-2,315,562, the hollow support mandrel is of metal and capable of being taken apart and formed by spaced sectors comprising apertures in which points are driven, around which points are stretched out threads forming the various crossed superimposed layers which are thereafter sewn by rows of stitches formed in the gaps between the sectors of the mandrel.
All the processes and apparatus described in these documents require a hollow mandrel since the connection of the superimposed layers by stitches necessarily requires introduction of a device in the mandrel for knotting the thread introduced from the exterior.
Moreover, the stitches are effected with needles having a flap valve or a closed eye which is delicate to use for fragile fibers which sometimes require a lapping of the thread.
Another process and device described in FR-A-2,408,676 employ on the other hand a solid mandrel composed of foam material in which sections of rigid threads, termed "picots", are implanted and around which the layers of threads are laid in two different directions and which constitute the threads of the third direction.
This process has various drawbacks. First of all, the "picots" must be previously subjected to a pre-rigidifying treatment, which increases their diameter, to permit their implantation.
Secondly, the "picots" which must become an integral part of the part to be produced must consequently be provided in a considerable number, on the order of several tens of thousands, implanted very close to one another, which is practically impossible with a machine.
Furthermore, in the case of a part having a complex shape whose surface forms corners or curves, the implantation of the neighboring "picots" which are excessively close together is very difficult to achieve without interference therebetween, and the very narrow passages they define do not permit an easy laying of the threads in even layers, which laying is found to be even impossible in the regions where the threads change orientation.
Lastly, the "picots" excessively close to one another behave imperfectly, in particular in the curved parts, and this results in defective homogeneity in the finished part.